For external works the existing gravel overburden was removed and the roof slab jetted clean with the roof joints then over-banded. In total 9 upstands were raised by 150mm, which was done by casting an extension of the existing upstand footprint to allow for the new access manways to be installed.
Following the installation of the manways, a new bitchumen roof membrane was installed to the central area of the roof slab. A liquid applied membrane was then installed to the perimeter upstand and to the access upstands. Geotextile was then installed over the new membrane and the overburden was reinstated using 40mm single sized gravel.
A GRP walkway was installed over the top of the gravel overburden to provide safe access to each of the manways, with and a perimeter handrail also installed along with 3 sets of access steps and concrete pads as a base to support these.
With the works ongoing, we were also asked to investigate 2no buried washout/drain valves which had been buried for a number of years. On investigation, it was found that the valves were still in situ but required replacements, so our contract was extended to carry out works to fully expose and install new valves along with access chambers.
In the first cell all of the internal joints, including the roof wall joint were prepped and over-banded as the existing seals had passed the serviceable life span. The outlet pipework was also prepped and coated with a polyurethane coating to prevent further corrosion. These works were then repeated on the second cell. Both cells were then cleaned and returned to service.
The external ladders were heavily corroded so these were removed and replaced with new stainless steel access ladders.